One of the most common errors in Lua is to misspell an identifier. If you have

local tests
print(test)

test will refer to a global. The AST contains this information. Many code styles, especially for modules, do not use globals much outside of local foo=foo imports. A visual highlight on test in

print(test)

would help people coding this way.

I don't know much about the DLTK, but Smalltalk has a cheap non-typed typo checker for method selectors. Translated to Lua, an occurrence of foo.testt is likely to be a typo unless somewhere else in the program any